Jeng-Rong Ho

Laser Patterning of Carbon-Nanotubes Thin Films and Their Applications

In 1960 T.H. Maiman successful fabricated the first laser and projected the first laser light. Fifty years later, lasers have found extensive applications in fields including energy, materials, communications, biotechnology and mechanical engineering. In the past several decades, continuous developments have improved the performance of lasers. From an initial wavelength of 694.3 nm, new lasers have been developed achieving wavelengths in the infrared (10.6 m) and ultraviolet (157 nm) spectra. Laser light has the several features which distinguish it from traditional light sources: directionality, brightness, monochromatity and coherence. These four characteristics have resulted in lasers having a profound influence on industrial development. In the following, the use of laser for material transfer will be briefly introduced.

Fabrication of carbon nanotube field emission cathodes in patterns by a laser transfer method

This study reports a novel approach, based on the method of laser induced pattern transfer, that can deposit patterned carbon nanotube (CNT) field emission cathodes on a variety of substrates at room temperature and in an ambient environment. Tests of emission characteristics of the fabricated cathodes present favourable emission current flux, low threshold electrical field, 1?mA?cm?2 current density at 3.3?V??m?1 for the 8??m thick film and good emission focusing. Well-defined CNT patterns with a feature size down to 10??m were produced using a mask. Compared with CNT film produced by printing based methods, higher density and better adhesion were achieved, without any post-treatment. Control of the film thickness can plainly be accomplished by adjusting the coated CNT film thickness on the support and the number of laser pulses. The fast deposition rate and high degree of feasibility of using the substrate, as well as the fabrication environment, render the proposed approach a potential method for low cost fabrication of precision patterns for CNT.

PEDOT:PSS/Graphene Nanocomposite Hole-Injection Layer in Polymer Light-Emitting Diodes

We report on effects of doping graphene in poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate), PEDOT:PSS, as a PEDOT:PSS/graphene nanocomposite hole injection layer on the performance enhancement of polymer light-emitting diodes (PLEDs). Graphene oxides were first synthesized and then mixed in the PEDOT:PSS solution with specifically various amounts. Graphenes were reduced in the PEDOT:PSS matrix through thermal reduction. PLED devices with hole-injection nanocomposite layer containing particular doping concentration were fabricated, and the influence of doping concentration on device performance was examined by systematically characterizations of various device properties. Through the graphene doping, the resistance in the hole-injection layer and the turn-on voltage could be effectively reduced that benefited the injection and transport of holes and resulted in a higher overall efficiency. The conductivity of the hole-injection layer was monotonically increased with the increase of doping concentration, performance indices from various aspects, however, did not show the same dependence because faster injected holes might alter not only the balance of holes and electrons but also their combination locations in the light-emitting layer. Results show that optimal doping concentration was the case with 0.03 wt% of graphene oxide.

A lattice Boltzmann approach for the non-Newtonian effect in the blood flow

Abstract A new lattice Boltzmann approach within the framework of D2Q9 lattice for simulating shear-thinning non-Newtonian blood flows described by the power-law, Carreau–Yasuda and Casson rheology models is proposed in this study. The essence of this method lies in splitting the complete non-Newtonian effect up into two portions: one as the Newtonian result and the other as an effective external source. This arrangement takes the advantage in remaining fixed relaxation time during the whole course of numerical simulation that can avoid the potential numerical instability caused by the relaxation time approaches to 1/2, an inherent difficulty in the conventional lattice Boltzmann methods using varying relaxation times for the non-Newtonian effect. Macroscopically, consistency of the proposed model with the equations of motion for the three target non-Newtonian models is demonstrated through the technique of Chapman–Enskog multi-scale expansion. The feasibility and accuracy of the method are examined by comparing with the analytical solutions of the two-dimensional Poiseuille flows based on the power-law and Casson models. The results show that the velocity profiles agree very well with those of analytical solutions and the error analyses demonstrate that the proposed scheme is with second-order accuracy. The present approach also demonstrates its superiority over the conventional lattice Boltzmann method in the extent of numerical stability for simulating the power-law-based shear-thinning flows. The straightforwardness in scheme derivation and implementation renders the present approach as a potential method for the complex non-Newtonian flows.

Characterizations of photoconductivity of graphene oxide thin films

Characterizations of photoresponse of a graphene oxide (GO) thin film to a near infrared laser light were studied. Results showed the photocurrent in the GO thin film was cathodic, always flowing in an opposite direction to the initial current generated by the preset bias voltage that shows a fundamental discrepancy from the photocurrent in the reduced graphene oxide thin film. Light illumination on the GO thin film thus results in more free electrons that offset the initial current. By examining GO thin films reduced at different temperatures, the critical temperature for reversing the photocurrent from cathodic to anodic was found around 187°C. The dynamic photoresponse for the GO thin film was further characterized through the response time constants within the laser on and off durations, denoted as τon and τoff, respectively. τon for the GO thin film was comparable to the other carbon-based thin films such as carbon nanotubes and graphenes. τoff was, however, much larger than that of the others. This...

Rapid Fabrication of Silver Nanowires through Photoreduction of Silver Nitrate from an Anodic-Aluminum-Oxide Template

A method for rapidly fabricating dense and high-aspect-ratio silver nanowires, with wire diameter of 200 nm and wire length more than 30 µm, is reported. The fabrication process simply involves filling the silver nitrate solution into the pores of an anodic-aluminum-oxide (AAO) membrane through capillary attraction and irradiating the dried template AAO membrane using a pulsed ArF excimer laser. Through varying the thickness and pore diameter of the employed AAO membrane, the primary dimensions of the targeted silver nanowires can be plainly specified; and, by amending the initial concentration of the silver nitrate solution and adjusting the laser operation parameters, laser fluence and number of laser pulses, the surface morphology and size of the resulting nanowires can be finely regulated. The wire formation mechanism is considered through two stages: the period of precipitation of silver particles from the dried silver nitrate film through the laser-induced photoreduction; and, the phase of clustering, merging and fusing of the reduced particles to form nanowires in the template pores by the thermal energy owing to photothermal effect. This approach is straightforward and takes the advantage that all the fabrication processes can be executed in an ambient environment and at room temperature. In addition, by the excellence in local processing that the laser possesses, this method is suitable for precisely growing nanowires.

Tunable liquid iris actuated using electrowetting effect

Abstract. A configuration for a tunable liquid iris, which consists simply of two immiscible liquids and two flat indium tin oxide (ITO) glass substrates, is proposed. The two immiscible liquids are transparent salt solution and opaque oil, respectively. The top ITO electrode was precoated with a 2-μm-thick polydimethylsiloxane film as the dielectric layer, while the surface of the bottom electrode was specially treated using ultraviolet irradiation to define specific hydrophilic regions. The iris aperture’s diameter could easily be regulated by varying the direct current bias voltages between the two electrodes. Results show that the aperture diameter can be continuously varied from 1.5 mm at the voltage-off state to 3.5 mm at a bias of 350 V. This liquid iris takes the advantages of low fabrication cost, fast response time, low-power consumption, and easy reversibility without the need of any mechanical movable parts.

A lattice Boltzmann model for electromagnetic waves propagating in a one-dimensional dispersive medium

A first-order extended lattice Boltzmann (LB) model with special forcing terms for one-dimensional Maxwell equations exerting on a dispersive medium, described either by the Debye or Drude model, is proposed in this study. The time dependent dispersive effect is obtained by the inverse Fourier transform of the frequency-domain permittivity and is incorporated into the LB evolution equations via equivalent forcing effects. The Chapman-Enskog multi-scale analysis is employed to ensure that proposed scheme is mathematically consistent with the targeted Maxwells equations at the macroscopic limit. Numerical validations are executed through simulating four representative cases to obtain their LB solutions and compare those with the analytical solutions and existing numerical solutions by finite difference time domain (FDTD). All comparisons show that the differences in numerical values are very small. The present model can thus accurately predict the dispersive effects, and demonstrate first order convergence. In addition to its accuracy, the proposed LB model is also easy to implement. Consequently, this new LB scheme is an effective approach for numerical modeling of EM waves in dispersive media.

Patterning of metallic electrodes on flexible substrates for organic thin-film transistors using a laser thermal printing method

We report on a laser thermal printing method for transferring patterned metallic thin films on flexible plastic substrates using a pulsed CO2 laser. Aluminium and silver line patterns, with micrometre scale resolution on poly(ethylene terephthalate) substrates, are shown. The printed electrodes demonstrate good conductivity and fulfil the properties for bottom-contact organic thin-film transistors. In addition to providing the energy for transferring the film, the absorption of laser light results in a rise in the temperature of the film and the substrate. This also further anneals the film and softens the plastic substrate. Consequently, it is possible to obtain a film with better surface morphology and with its film thickness implanted in part into the plastic surface. This implantation reveals excellent characteristics in adhesion and flexure resistance. Being feasible to various substrates and executable at ambient temperatures renders this approach a potential alternative for patterning metallic electrodes.

Fast Fabrication of Silver Nanowires Using Photothermal Reduction from an Anodic Aluminum Oxide Template

A method for fabricating silver nanowires is reported. Based on irradiation from a pulsed CO 2 laser with a high repetition rate, dense and high aspect ratio silver nanowires are grown from an anodic aluminum oxide template within several seconds. The morphology and composition of the fabricated nanostructures are characterized, and the detailed mechanisms for the growth are addressed. This method is straightforward and takes the advantage of heating the template and reducing nanowires locally, which is suitable for low temperature fabrication.